The invention described herein relates to vermiculite compositions having enhanced volume increase upon thermal exfoliation and to products formed therewith. Specifically, the vermiculite composition is composed of particulate vermiculite having a specific gravity of from 2.5 g/cm.sup.3 to 2.9 g/cm.sup.3. The subject composition can be formed into sheet products, utilized as a component in cement compositions or be thermally exfoliated to provide an exceptionally high yield material.
Vermiculite ore is composed of micaceous minerals, chemically identified as a hydrated magnesium-aluminum-iron silicates and characterized by a platelet structure which exfoliate or expand in a single direction when subjected to high temperatures treatment. The vermiculite increases several times its original size and traps within itself thousands of minutely sized air cells. The minerals occur naturally in an unexpanded state, and are mined in conventional manner.
The vermiculite ore, as mined, is associated with other minerals and undesirables. The crude vermiculite ore normally contains about 15 to 30 weight percent commercially desired vermiculite material. This crude ore is beneficiated by various known surface chemistry techniques to separate the gangue from the expandable material. For example, the crude ore is normally washed with water to remove clays and other associated fines. The remaining material is subjected to screening (with optional prior crushing to reduce particle size) to separate sand, rock and other debris from the platelet materials which contain non-expandable minerals (mica, etc.) and expandable materials. These are separated by treating the material with a surfactant and subjecting it to flotation or wet concentrating table techniques to recover vermiculite ore concentrate. Ore concentrate is generally composed of 85 to 99 weight percent expandable vermiculite ore having a range of specific densities depending on its platelet configuration.
The naturally occurring ore and the ore concentrate are composed of a mixture of vermiculite mineralogical species having a broad spectrum of densities ranging from less than 1.9 g/cm.sup.3 to over 3.0 g/cm.sup.3.
Vermiculite minerals are well known for their capacity to undergo expansion to several times original volume when subjected to elevated temperatures. The process of expanding vermiculite ore concentrate by heating is referred to as thermal exfoliation. This process is conventionally achieved by a continuous procedure in which vermiculite concentrate are fed into an expansion kiln or oven, heated rapidly to temperatures in the range of 1100.degree. F. to 1900.degree. F., and the resultant expanded vermiculite product removed from the heating zone as additional unexpanded material is introduced.
It is common in the art of thermal exfoliation of vermiculite to refer to the "yield" obtained by exfoliation. This yield is the volume of expanded vermiculite, when loosely accumulated without compression of the expanded particles, obtained from an initial weight of unexpanded material. Since expanded vermiculite is generally sold on a volume basis, it proves economically advantageous to the producer to maximize the yield. The expanded vermiculite has a wide variety of well known uses, including use as an aggregate in plaster and concrete formulations, a thermal insulator, an extender in fertilizers and soil mixes, an inert carrier for chemicals, such as insecticides herbecides, nutrients and the like, a packaging material, and as roughage or filler in animal feed.
Various methods have been proposed to increase the yield on thermal exfoliation. U.S. Pat. No. 3,062,753 to Hayes teaches that yield may be increased by subjecting the ore to ion exchange pre-treatment. U.S. Pat. No. 3,686,134 to Robinson teaches a method requiring preheating the ore concentrate at low temperatures under a saturated moisture atmosphere prior to subjecting the concentrate to thermal exfoliation. Canadian 1,241,153 to Hindman teaches that by applying vegetable oil to the vermiculite ore concentrate one achieves higher yields upon thermal exfoliation. Because of the elaborate demands of these processes, they are not economically effective with respect to the product (e.g. packing, feed, filter, etc.) achieved.
Vermiculite ore concentrates have also been used to form flexible intumescent sheet products, as described in U.S. Pat. Nos. 3,916,057; 4,305,992; and 4,385,135, which teachings are incorporated herein by reference. The intumescent sheet may be used to mount ceramic cores in catalytic converters. In such application, the intumescent sheet expands in situ upon initial subjection to the hot exhaust gases to hold the ceramic core in place within the canister body. Such sheet products are generally formed from conventional vermiculite ore concentrate and, thus, have varying thermal-mechanical properties depending on the natural ore's composition. In certain instances, the concentrate is treated to ion exchange treatment to enhance the properties of the concentrate. It is highly desired to provide an intumescent sheet product having low initial expansion temperature and a very high maximum expansion to provide a sheet of uniform thermal-mechanical characteristics.
It is highly desired and an object of the present invention to provide a vermiculite composition which is capable of exhibiting enhanced yields upon heating at elevated temperature.
It is highly desired and an object of the present invention to provide an economical process for forming a vermiculite composition capable of exhibiting high yields.
It is highly desired and an object of the present invention to produce shaped products, such as sheets and the like from a vermiculite composition, which can exhibit enhanced expansions and desirable thermal-mechanical properties upon subjection to elevated temperatures.